In U.S. Pat. No. 5,127,021 (the '021 Patent) and in U.S. patent application Ser. No. 07/965,227 (the '227 Application), both of which are incorporated herein by reference, TV systems using spread spectrum were disclosed. The present invention is an improvement of the earlier inventions, providing higher resistance to channel impairments as well as the ability to be employed in single-frequency networks (SFNs).
The U.S. is in the process of developing standards for over-the-air broadcasting of high-definition television (HDTV), also called Advanced Television (ATV).
Because substantial data compression is required in order to permit transmission in the same 6-MHz channel that is used for the current system (NTSC), system proponents have proposed all-digital systems, using both digital source coding and digital channel coding. The source-coding methods are now highly developed, but the proposed channel-coding methods, which were developed for wire-line transmission, are not. The over-the-air channel is a particularly hostile environment for digital transmission, plagued as it is by multipath transmission, noise, and interference. As a result, there is considerable doubt as to whether the goals of convenience, reliability, efficient use of spectrum, and provision for a range of receivers of different price and performance will be achieved.
Cable companies are also planning to use ATV technology, but appear to be more interested in transmitting a multiplicity of programs of today's definition in each cable channel rather than transmitting one HDTV signal per channel. Cable channel characteristics are somewhat different from those of the over-the-air channel, but many of the same problems are present. The need to serve a range of receivers of different price/performance is common to both kinds of service. The need to optimize performance for reception conditions is also common to both since cable does not provide equal-quality signals to all receivers.
The overall object of the invention is to overcome the limitations of existing and proposed TV transmission systems, particularly in connection with broadcasting of coded HDTV signals. In systems that use a single centralized transmitter, the invention permits good performance in spite of multipath (ghosts or echoes), noise, and interference. It makes efficient use of spectrum and optimizes picture quality at each receiver in the light of its individual reception conditions. It permits the use of relatively inexpensive receivers (typically with small screens) in applications where maximum image quality is not required. In single-frequency networks, in which the reception area of a station is served by a cellular array of low-power transmitters, all operating on the same frequency, its remarkable resistance to multipath permits satisfactory operation in spite of the presence of many strong ghosts. In all applications, the invention permits the use of less sophisticated antennas and simpler channel equalizers than are required for currently proposed all-digital systems.